Cold Arbor Update 6: More Drivetrain and the Saw Actuator

Sunday, January 24th, 2010 @ 9:14 | Bots, Cold Arbor, Project Build Reports

Hey, where the heck did January go? There’s only a week left!

That means there’s only 3 weeks left until Motorama 2010. Time to pump up the volume on Arbor work. While awaiting more waterjetting time, I finished most of the more complex machined parts for the robot.

I’ve grown into the habit of making my own linear actuators for the robots. Acme nuts and threaded rod are cheap on the surplus market, and I can tune the characteristics of the actuator to suit. I prefer linear actuators and linkages for moving robot assemblies around over direct torque couplings on the end of a gearbox, because it’s a bit easier to make a part strong in tension or compression rather than shear, and the leadscrew isolates the actuator motor from torque shock. Überclocker features a prominent exception to this because the clamp arm needs almost 150 degrees of travel, which is much more difficult to accomplish with a linkage and linear actuators.

Anyway, the saw arm of the robot only needs to travel about 60 degrees, which a single 3 link planar linkage can easily do. A clear image of the actuator is visible in this early design rendering of Arbor. It’s essentially a scaled up version of Clocker’s top clamp arm actuator – a leadscrew nut trapped between two thrust bearings.

To make the actuator body, I needed two big chunks of 2″ x 2.5″ rectangular aluminum bar. I had the bar, but both bandsaws at MITERS were simultaneously down.

Well that sucks. People need to stop accidentally cutting hardened steel on them or something.

The time of day I usually work on these things coincides with absolutely nothing else open on campus that houses a machine tool capable of cutting metals. So, I had to create a hackaround…

Bridgeport lovers and shop instructors avert thine eyes.

Wow, what the hell is that?

It’s a 6 inch milling cutter (with R8 arbor!) that a fellow MITERer picked up to cut a bunch of deep slits in steel. So, I’m not fundamentally doing anything worse, but it’s still one of those exercises that has the potential to destroy property and cause personal injury.

So, with the machine in low gear, a constant stream of Tap Magic, and everything cranked down as tight as I could manage, I plowed the cutter through just under 2 inches of aluminum leaving a roughly .02″ thick edge uncut. This was done mostly to keep the block from being pitched through a window as soon as it fell off the saw.  The whole process took a minute, and…

…left a brilliantly clean finish, almost fly-cutter-like in appearance. To remove the block, I just ripped it off with some vise grips.

Alright, so that was the fun part. Here’s a leap of faith and some finished actuator housings. No other special machining hacks were involved in the making of these parts.

Well, maybe one. Bridgeports have a Z-axis knee handle that can detach from the machine and be stored elsewhere. This is so you don’t accidentally run into it and get OSHA on your case, or move the Z-axis setting. Unfortunately, they have a bad habit of detaching themselves from their drive splines, especially when you’re trying to crank the Z as hard as you can. This has resulted in me clocking myself with the cast aluminum handle in the forehead at least once.

After having the handle fly off too many times, I finally got pissed off enough and put a shaft collar on the handle shaft so the stupid thing doesn’t come off. Ever. I don’t care if Bridgeport made them this way for a reason.

The first tool casualty in a long time comes in the form of me dropping a fully loaded boring head after finishing one of the actuator halves. It landed on the point of the tool, and so the entire tip of the boring bar cracked off.

Sad face. Time to start checking Ebay again?!

Fortunately, I was about to continue with a spare.

There’s something weird about the output gear in this actuator. It’s not a gear. It’s actually a #25 sprocket!

I found out that surplus chain and sprockets can literally be an order of magnitude cheaper than using spur gears. You can’t seem to find industrial 24 or 20 pitch metal spur gears for under $20 to $25 a pop. I was going to have to pay out almost $90 in spur gears alone for the two actuators in the robot.

But short runs of chain can perform the same duties. Surplus Center’s chain and sprocket selection was just too cheap to not explore these options. So, I decided to make the motor-to-leadscrew connection using chain instead. Enough sprockets and chain to build both actuators ran just under $11.

The 14 tooth sprocket shown here is squeeze-fitted onto a 1/2″-10 Acme nut, which, incidentally, is also Surplus Center hardware.There will be an 11 tooth sprocket mounted on the drill motor output shaft.

I heart Surplus Center.

The sprocket was actually once an independent power transmission component. To remove the “sprocket” part, I bored it to death on the the Old Mercedes. One cut at the diameter of the hub, and the outer ring with the teeth just pops off and lands on the tool.

While I had the machines still set up, I popped off these protoforms of the rear drive hubs. The flanges will have a 3 point bolt circle drilled into them later, and two flats will be machined near the retaining ring groove in order to make room for custom D-bore sprockets, just like on Überclocker.

Mounted on their respective gearboxen. The smallnubs will fit into bronze bushings in the side of the robot.

The Scene™ as of yesterday. Lots of things “almost, kind of, sort of” done, but not really.

Time to go catch up on waterjetting so I can continue building…

 

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